1. Technical Field
The present invention relates to an apparatus and method for non-destructive testing of stress within solid materials using the LCR ultrasonic technique. The improved technique and apparatus provides a more accurate measurement.
2. Description of the Related Art
The non-destructive testing for stress in metals has long been recognized as an important method for evaluating metal components to predict both the failure location and rate, to identify stressed components prior to failure, and many other safety related considerations. Non-destructive testing is used extensively in a wide variety of industries including the aviation, automotive, petroleum, and chemical industries, and various construction and structural related fields. The use of non-destructive testing on specific components ranges from the testing of steel turbine blades in jet engines to steel support beams in bridges and other large structures. The benefit of non-destructive testing is self-evident. Components in use can be tested to determine the stress levels in the components without destroying the components.
Techniques such as x-ray diffraction and Barkhausen noise analysis have been successfully applied for nondestructive stress measurements. While the x-ray techniques are quite reliable in their measurements, they measure stress in only the top few angstroms of the surface, and the results may not be indicative of internal stresses. The Barkhausen method is based on small changes in magnetic permeability due to stress. Application of the Barkhausen method is limited to electrical conducting materials. It has been available for a number of years, and has not been shown to give generally reliable results.
The LCR ultrasonic technique indicates stress through the acoustoelastic principle where small variations in strain affect the wave speed. By measuring the wave speed (or travel-time between known points) the change in stress can be calculated. Other material variations such as texture and temperature also affect the travel-time. The investigator using the LCR method must be aware of these other effects so that the best data indicative of stress variation is obtained.
The relationship of measured LCR wave travel-time change and the corresponding uniaxial stress is given by:   Δσ  =            E              Lt        0              ⁢          (              t        -                  t          0                -                  Δ          ⁢                      xe2x80x83                    ⁢                      t            T                              )      
where xcex94"sgr" is change in stress, E is Young""s modulus, and L is the acoustoelastic constant for longitudinal waves propagating in the direction of the applied stress field, as given in Table 1. Travel-time change (xcex94t) is the measured travel-time (t) minus the reference travel-time (t0). The reference travel-time t0 is a property of the probe sensor spacing for an assumed stress free material.
Temperature induced speed changes occur both in the material being investigated and in the probe material. The relationship of wave speed and temperature (dc/dT) is given by:             ⅆ      c              ⅆ      T        =            k      T        ⁢          m              s        -                  xc2x0          ⁢                      xe2x80x83                    ⁢                      C            .                              
where kT is the constant for a particular material, as given in Table 2.
The effect of temperature on travel-time will be       Δ    ⁢          xe2x80x83        ⁢          t      T        =      d                  k        T            ⁢      Δ      ⁢              xe2x80x83            ⁢      T      
where d is the travel distance in the material and xcex94T is the measured temperature change. Obviously, the temperature effect for the PMMA wedge material is greater than that of the steel. Where data are collected under moderately uniform temperature conditions, the temperature effect, xcex94t, can be ignored. For large temperature variations, the temperature of the wedge can be monitored with a thermocouple. The distance of wave travel in the wedge can be measured and a suitable correction in the travel-time can be made.
Texture as typically encountered in cold-rolled plates and other structural shapes can have a significant affect on the wave speed. While the affect of texture on the LCR wave speed is less than that encountered by the shear waves often used in acoustic-birefringence stress measurements, there still is concern about the effects.
Special data collection procedures may be used to minimize the effects of texture. In many items where stress is a concern, the texture may be uniform throughout. In these cases, LCR travel-times taken with the probe always at the same orientation relative to the geometry of the item may be free of texture variation. In this case, the major effect may be stress. This has been found to be true for plates and welded plates. However, there is a need for more data on additional structures and shapes before this assumption may be more widely made.
Ideally, the LCR pulse is a true, nondispersive wave travelling at the longitudinal wave speed of the material. There are shape and material effects, however, that can cause dispersion of the wave. In many of these cases, the wave can still be used for stress measurement by the careful operator, and by choosing the proper probe.
Wave-guide effects are one of the most serious causes of dispersion. These occur in plates and pipes when the wavelength of the wave approaches some fraction of the thickness. Typically, when the ratio of plate thickness to wavelength is ten or above, there is no risk of any waveguide effect. Satisfactory results have been obtained with ratios of five, however. Texture effects, discussed above, and grain boundary scattering also affects the pulse shape. Waveguide effects are the easiest to eliminate due to knowledge of the geometry. Texture may be evaluated with a contact shear wave acting across the thickness. Grain boundary scattering may be evaluated with attenuation measurements also across the boundary. There are no data yet on acceptable ranges for LCR stress measurement, however.
Choosing the proper reference location within the LCR pulse can enable the collection of reliable data. Typically, the second positive zero crossing at the first arrival of the pulse is used as this reference. In nondispersive conditions, this location is easy to identify at all pulse arrivals. Under dispersive conditions, however, identification may be more difficult. In difficult circumstances, identification can be aided by sliding a receiver probe along the travel path and observing the change of shape.
Ultrasonic stress measurement techniques have been developed in the past. Some use longitudinal waves, but they have not met with success due to the absence of a method for accurately controlling the coupling state between the probe and the item being inspected. Others use shear (SH) and/or Rayleigh waves which are well known to be less sensitive to stress than is the LCR wave. Other significant workers in the field of ultrasonic stress measurement include R. B. Thompson at Iowa State University, Eckhardt Schneider at the Fraunhofer Institute in Saarbrucken, Germany, Teodore Leon-Salamanca and Eugene Reinhart at Reinhart and Associates in Austin, Tex. and Ray Schramm and Van Clark at the National Institute for Science and Technology in Boulder, Colo.
Accordingly, a need exists for a non-destructive testing method and apparatus to accurately indicate the internal stresses of metal. The method and apparatus should accurately control the coupling state between the probe and the item being inspected and take into account or avoid various interference factors, thus providing accurate and reliable stress measurements.
The LCR ultrasonic technique is a unique nondestructive method for evaluating stress levels and other mechanical property variations in engineering components, structures and materials. The inspection is accomplished with a newly designed apparatus incorporating an LCR ultrasonic probe, a variable force application device and a mechanism for attachment of the probe to the item being evaluated.
The new feature of the LCR technique is the ability to apply an even, linear variable force to the interface of the LCR probe and the item being inspected. The force is equal at the interfaces of the two or more receiving probes since the single force applicator is located equidistant between the interfaces. Further, the magnitude of the force can be established through the use of a measurement device such as a pressure gauge. This feature enables the reliable and repeatable measurement of 0.0004% or better on the LCR travel-time changes. The LCR probe operates in a send-receive mode, using one transmitter (T) and either one or more receivers (R1 and/or Rn). Both the send and receiving probes are on one side of the material. The LCR wave is excited at approximately the first critical angle +/xe2x88x922 degrees for the wedge and material combination. The pulse travels from the transmitter to the receivers as a bulk critically refracted longitudinal (LCR) wave and encounters the stress effect in its path. Since the LCR wave propagation is just beneath the surface, the stress and other material property variations within its penetration path affect the speed. Also, surface conditions have little affect on the wave travel. Moreover, frequency variation and analysis techniques may establish stress and other property gradients existing below the surface.
The LCR stress measurement process has been demonstrated in lab and field applications in railroad rail, welded steel plates, a turbine disk, a compressor rotor, rolled aluminum plates, rolled steel plates, titanium plates and ductile cast iron samples. A prototype LCR stress measurement apparatus has been developed and demonstrated.
Generally speaking, the requirement for the contact area during the inspection is that it must be free of dirt, water, oil, scale and other loose debris that can affect the probe contact. Ordinary metal scale does not affect the LCR data, provided that it is tightly adhered to the plate or pipe. Ultrasonic instrumentation preferable for collecting LCR data are a typical commercial pulser/receiver and a computer data acquisition system with a high speed digitizing board, or a digital oscilloscope. Arrival time resolution of 0.1 ns or better is preferable for the instrumentation. The board and suitable software can be conveniently fitted into expansion slots on a PC or Laptop computer. Since temperature can affect the travel-time data, these data also may be collected during the test. Stress changes may then be calculated from the arrival times, using the appropriate formulas.
Judgement of stress change or stress fields would be based on the deviation of the observed travel time from some previously established zero stress norm. The zero stress norms may be established with prior data on the material being inspected, or on comparison with a known stress free region in the material.
The distinct advantage of the LCR ultrasonic technique used by the apparatus described here is that the frame and force application feature enables the accurate collection of travel-time data. These data will enable the establishment of stress variations in the plane of the plate or pipe, and with depth.
The above, as well as additional features and advantages of the present invention will become apparent in the following written detailed description.